1. Field of the Invention
The invention relates to a method for magnetic resonance imaging of a body placed in a stationary and substantially homogeneous main magnetic field, which a method includes the application of an excitation radio-frequency pulse (RF-pulse) for excitation of nuclear dipole moments in at least a portion of the body, the application of a plurality of refocusing RF-pulses following said excitation RF-pulse and of gradient magnetic fields for generating magnetic resonance signals a comprising position dependent information and the measurement of magnetic resonance signals which follow at least a number of the refocusing RF-pulses. The invention also relates to an apparatus for magnetic resonance imaging using such a method. Such a sequence of RF-pulses is, for example, a CPMG-sequence (Carr-Purcell-Meiboom-Gill sequence) which generates multiple nuclear magnetic resonance echo signals (NMR-signals) following the refocusing RF-pulses.
2. Description of the Related Art
Such a method for imaging is known from EP-A 0 175 184, which corresponds to U.S. Pat. No. 4,818,940. As indicated in that document, a selection of a portion of the body is made by the application of a gradient magnetic field during the application of the excitation RF-pulse. This results in the excitation of a slice of the body in which slice the Larmor frequency of a selected nucleus type in the magnetic field corresponds to the frequency of the RF-pulse. The slice selection gradient magnetic field is also applied during application of the refocusing RF-pulses. For position determination of the magnetic resonance signals, a gradient magnetic field with the gradient in a first direction within the slice is applied in the interval between the refocusing RF-pulses and the measurements of the nuclear magnetic resonance (NMR) signals for phase encoding of the NMR-signals. During the measurement a second gradient magnetic field, with its gradient in a second direction within the slice and perpendicular to the first direction provides frequency encoding of the NMR-signals.
In case a volume is selected as the excited portion of the body, a slice selection gradient may be absent during the RF-pulses. In addition to first and second gradient magnetic fields, a third gradient magnetic field with its gradient perpendicular to the first and second gradient magnetic fields is applied in between the RF-pulses and the measurements of the NMR-signals for additional phase encoding in that direction.
A disadvantage of such a sequence is that, due to spin-spin relaxation, the T.sub.2 -decay, the magnitude of the spin-echo signals decreases. So, contributions from the same region of the body portion are measured as signals of different size. In the resulting image, this leads to undesirable image errors such as blurring and ringing artefacts. As the total measured signal contains contributions of substances with different T.sub.2 value, removal of the artefacts by weighting the signals with a time dependent weight is not possible.